A) NORMALITY B) GM/LITRE C) MOLARITY AND D) PERCENTAGE
THEORY:
Reaction H₂SO4 + 2NaOH ——>Na₂SO4+ 2H₂O
Ionically, 2H+ + 20H– ——-> 2H₂O
Bench acids are usually much stronger than the standard alkali supplied. The direct titration between the strong acid and the dilute alkali does not give accurate results. Therefore, the strong acid should be diluted to the required extent such that the diluted acid and the alkali neutralize more or less in equal volumes. Then by knowing the strength of the diluted acid, we can determine the strength of the original bench acid.
APPARATUS :
1) Burette
2) Pipette
3) Conical flask
4) Burette stand
5) Clamps
6) Measuring flask
CHEMICALS:
1) Phenolphthalein or methyl orange indicator
PROCEDURE:
i) Wash the whole apparatus with water.
ii) Rinse the burette with the bench sulphuric acid two times, and then fill it up with this acid itself. See that the acid solution is continuous from zero mark down to the nipple without any gap or air bubble.
iii) Rinse the pipette with the standard alkali solution, and pipette out 10ml of the alkali into the clean conical flask, and add one or two drops of phenolphthalein indicator when the solution turns pink.
iv) Now, add acid drop by drop to the alkali till the solution becomes just colorless [you will find that a few drops of the bench acid can neutralize the whole of the alkali]. So, this is rough titration.
Rough titration reading :
Suppose, 0.4 ml of bench H₂SO4 = 10 ml of alkali
We know,
V₁ x N₁ = V₂ × N₂
:. N₁ = 10/0.4 x N₂ = 25 N₂
It means, the bench acid is 25 times stronger than the alkali. Hence the acid should be diluted 25 times.
Dilution :
Suppose, a 100 ml measuring flask is provided for dilution.
Then, the volume of the bench H₂SO4 required for dilution up to 100ml = 100/25 =4ml
v) Hence, drop 4 ml bench acid from the burette into the 100 ml measuring flask. Add water till it is about half full. Swirl the flask gently for a while, and make up to the volume by the addition of water. Stopper the flask, and shake well to make it homogeneous.
vi) Remove the bench acid from the burette, wash it thoroughly with water, rinse it 2 times with diluted acid, and then fill it with the same diluted acid. Titrate the diluted acid against the standard alkali as usual by using phenolphthalein as the indicator.
RESULTS:
Volume of bench acid taken for dilution = 4 ml
Solution taken in the burette = diluted acid
Alkali taken for each titration = 10 ml
Indicator used = Phenolphthalein
End point = from pink to colorless
Normality of standard alkali =N/10 (f = 0.96)
BURETTE READING:
| No. of obs. | Initial reading | Final reading | Vol of acid (ml) | Mean |
| 1 | 10.7—-> | (approx) | ||
| 2 | 10.5 | |||
| 3 | 10.4 | 10.4 | ||
| 4 | 10.4 |
CALCULATIONS :
Diluted acid Alkali
V₁ = 10.4 ml V₂ = 10ml
N₁ = ? N₂ = N / 10(f = 0.96)
From the normality equation,
N₁ = 10 × N × 0.96 / 10.4 x 10 =N / 10 (f=0.923)
Since, 4 ml of bench H₂SO4 are diluted to 100 ml.
4 ml of bench H₂SO4 = 100 ml of diluted acid
Applying the normality equation again.
Normality of bench H₂SO4 = 100xNx0.923 / 4×10
= 2.307 N
Strength in gm / liter = Normality x Equivalent wt.
= 2.307×49
= 113.043 gm / liter
Molarity = gm/liter / mol.wt
= 113.043 / 98
= 1.1535M.
Strength in % by (w/v) = 113.043 / 10
= 11.3043gm / 100ml.